1// SPDX-License-Identifier: GPL-2.0
  2/*
  3 * Manage cache of swap slots to be used for and returned from
  4 * swap.
  5 *
  6 * Copyright(c) 2016 Intel Corporation.
  7 *
  8 * Author: Tim Chen <tim.c.chen@linux.intel.com>
  9 *
 10 * We allocate the swap slots from the global pool and put
 11 * it into local per cpu caches.  This has the advantage
 12 * of no needing to acquire the swap_info lock every time
 13 * we need a new slot.
 14 *
 15 * There is also opportunity to simply return the slot
 16 * to local caches without needing to acquire swap_info
 17 * lock.  We do not reuse the returned slots directly but
 18 * move them back to the global pool in a batch.  This
 19 * allows the slots to coaellesce and reduce fragmentation.
 20 *
 21 * The swap entry allocated is marked with SWAP_HAS_CACHE
 22 * flag in map_count that prevents it from being allocated
 23 * again from the global pool.
 24 *
 25 * The swap slots cache is protected by a mutex instead of
 26 * a spin lock as when we search for slots with scan_swap_map,
 27 * we can possibly sleep.
 28 */
 29
 30#include <linux/swap_slots.h>
 31#include <linux/cpu.h>
 32#include <linux/cpumask.h>
 33#include <linux/vmalloc.h>
 34#include <linux/mutex.h>
 35#include <linux/mm.h>
 36
 37static DEFINE_PER_CPU(struct swap_slots_cache, swp_slots);
 38static bool	swap_slot_cache_active;
 39bool	swap_slot_cache_enabled;
 40static bool	swap_slot_cache_initialized;
 41static DEFINE_MUTEX(swap_slots_cache_mutex);
 42/* Serialize swap slots cache enable/disable operations */
 43static DEFINE_MUTEX(swap_slots_cache_enable_mutex);
 44
 45static void __drain_swap_slots_cache(unsigned int type);
 46static void deactivate_swap_slots_cache(void);
 47static void reactivate_swap_slots_cache(void);
 48
 49#define use_swap_slot_cache (swap_slot_cache_active && \
 50		swap_slot_cache_enabled && swap_slot_cache_initialized)
 51#define SLOTS_CACHE 0x1
 52#define SLOTS_CACHE_RET 0x2
 53
 54static void deactivate_swap_slots_cache(void)
 55{
 56	mutex_lock(&swap_slots_cache_mutex);
 57	swap_slot_cache_active = false;
 58	__drain_swap_slots_cache(SLOTS_CACHE|SLOTS_CACHE_RET);
 59	mutex_unlock(&swap_slots_cache_mutex);
 60}
 61
 62static void reactivate_swap_slots_cache(void)
 63{
 64	mutex_lock(&swap_slots_cache_mutex);
 65	swap_slot_cache_active = true;
 66	mutex_unlock(&swap_slots_cache_mutex);
 67}
 68
 69/* Must not be called with cpu hot plug lock */
 70void disable_swap_slots_cache_lock(void)
 71{
 72	mutex_lock(&swap_slots_cache_enable_mutex);
 73	swap_slot_cache_enabled = false;
 74	if (swap_slot_cache_initialized) {
 75		/* serialize with cpu hotplug operations */
 76		get_online_cpus();
 77		__drain_swap_slots_cache(SLOTS_CACHE|SLOTS_CACHE_RET);
 78		put_online_cpus();
 79	}
 80}
 81
 82static void __reenable_swap_slots_cache(void)
 83{
 84	swap_slot_cache_enabled = has_usable_swap();
 85}
 86
 87void reenable_swap_slots_cache_unlock(void)
 88{
 89	__reenable_swap_slots_cache();
 90	mutex_unlock(&swap_slots_cache_enable_mutex);
 91}
 92
 93static bool check_cache_active(void)
 94{
 95	long pages;
 96
 97	if (!swap_slot_cache_enabled || !swap_slot_cache_initialized)
 98		return false;
 99
100	pages = get_nr_swap_pages();
101	if (!swap_slot_cache_active) {
102		if (pages > num_online_cpus() *
103		    THRESHOLD_ACTIVATE_SWAP_SLOTS_CACHE)
104			reactivate_swap_slots_cache();
105		goto out;
106	}
107
108	/* if global pool of slot caches too low, deactivate cache */
109	if (pages < num_online_cpus() * THRESHOLD_DEACTIVATE_SWAP_SLOTS_CACHE)
110		deactivate_swap_slots_cache();
111out:
112	return swap_slot_cache_active;
113}
114
115static int alloc_swap_slot_cache(unsigned int cpu)
116{
117	struct swap_slots_cache *cache;
118	swp_entry_t *slots, *slots_ret;
119
120	/*
121	 * Do allocation outside swap_slots_cache_mutex
122	 * as kvzalloc could trigger reclaim and get_swap_page,
123	 * which can lock swap_slots_cache_mutex.
124	 */
125	slots = kvcalloc(SWAP_SLOTS_CACHE_SIZE, sizeof(swp_entry_t),
126			 GFP_KERNEL);
127	if (!slots)
128		return -ENOMEM;
129
130	slots_ret = kvcalloc(SWAP_SLOTS_CACHE_SIZE, sizeof(swp_entry_t),
131			     GFP_KERNEL);
132	if (!slots_ret) {
133		kvfree(slots);
134		return -ENOMEM;
135	}
136
137	mutex_lock(&swap_slots_cache_mutex);
138	cache = &per_cpu(swp_slots, cpu);
139	if (cache->slots || cache->slots_ret)
140		/* cache already allocated */
141		goto out;
142	if (!cache->lock_initialized) {
143		mutex_init(&cache->alloc_lock);
144		spin_lock_init(&cache->free_lock);
145		cache->lock_initialized = true;
146	}
147	cache->nr = 0;
148	cache->cur = 0;
149	cache->n_ret = 0;
150	/*
151	 * We initialized alloc_lock and free_lock earlier.  We use
152	 * !cache->slots or !cache->slots_ret to know if it is safe to acquire
153	 * the corresponding lock and use the cache.  Memory barrier below
154	 * ensures the assumption.
155	 */
156	mb();
157	cache->slots = slots;
158	slots = NULL;
159	cache->slots_ret = slots_ret;
160	slots_ret = NULL;
161out:
162	mutex_unlock(&swap_slots_cache_mutex);
163	if (slots)
164		kvfree(slots);
165	if (slots_ret)
166		kvfree(slots_ret);
167	return 0;
168}
169
170static void drain_slots_cache_cpu(unsigned int cpu, unsigned int type,
171				  bool free_slots)
172{
173	struct swap_slots_cache *cache;
174	swp_entry_t *slots = NULL;
175
176	cache = &per_cpu(swp_slots, cpu);
177	if ((type & SLOTS_CACHE) && cache->slots) {
178		mutex_lock(&cache->alloc_lock);
179		swapcache_free_entries(cache->slots + cache->cur, cache->nr);
180		cache->cur = 0;
181		cache->nr = 0;
182		if (free_slots && cache->slots) {
183			kvfree(cache->slots);
184			cache->slots = NULL;
185		}
186		mutex_unlock(&cache->alloc_lock);
187	}
188	if ((type & SLOTS_CACHE_RET) && cache->slots_ret) {
189		spin_lock_irq(&cache->free_lock);
190		swapcache_free_entries(cache->slots_ret, cache->n_ret);
191		cache->n_ret = 0;
192		if (free_slots && cache->slots_ret) {
193			slots = cache->slots_ret;
194			cache->slots_ret = NULL;
195		}
196		spin_unlock_irq(&cache->free_lock);
197		if (slots)
198			kvfree(slots);
199	}
200}
201
202static void __drain_swap_slots_cache(unsigned int type)
203{
204	unsigned int cpu;
205
206	/*
207	 * This function is called during
208	 *	1) swapoff, when we have to make sure no
209	 *	   left over slots are in cache when we remove
210	 *	   a swap device;
211	 *      2) disabling of swap slot cache, when we run low
212	 *	   on swap slots when allocating memory and need
213	 *	   to return swap slots to global pool.
214	 *
215	 * We cannot acquire cpu hot plug lock here as
216	 * this function can be invoked in the cpu
217	 * hot plug path:
218	 * cpu_up -> lock cpu_hotplug -> cpu hotplug state callback
219	 *   -> memory allocation -> direct reclaim -> get_swap_page
220	 *   -> drain_swap_slots_cache
221	 *
222	 * Hence the loop over current online cpu below could miss cpu that
223	 * is being brought online but not yet marked as online.
224	 * That is okay as we do not schedule and run anything on a
225	 * cpu before it has been marked online. Hence, we will not
226	 * fill any swap slots in slots cache of such cpu.
227	 * There are no slots on such cpu that need to be drained.
228	 */
229	for_each_online_cpu(cpu)
230		drain_slots_cache_cpu(cpu, type, false);
231}
232
233static int free_slot_cache(unsigned int cpu)
234{
235	mutex_lock(&swap_slots_cache_mutex);
236	drain_slots_cache_cpu(cpu, SLOTS_CACHE | SLOTS_CACHE_RET, true);
237	mutex_unlock(&swap_slots_cache_mutex);
238	return 0;
239}
240
241int enable_swap_slots_cache(void)
242{
243	int ret = 0;
244
245	mutex_lock(&swap_slots_cache_enable_mutex);
246	if (swap_slot_cache_initialized) {
247		__reenable_swap_slots_cache();
248		goto out_unlock;
249	}
250
251	ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "swap_slots_cache",
252				alloc_swap_slot_cache, free_slot_cache);
253	if (WARN_ONCE(ret < 0, "Cache allocation failed (%s), operating "
254			       "without swap slots cache.\n", __func__))
255		goto out_unlock;
256
257	swap_slot_cache_initialized = true;
258	__reenable_swap_slots_cache();
259out_unlock:
260	mutex_unlock(&swap_slots_cache_enable_mutex);
261	return 0;
262}
263
264/* called with swap slot cache's alloc lock held */
265static int refill_swap_slots_cache(struct swap_slots_cache *cache)
266{
267	if (!use_swap_slot_cache || cache->nr)
268		return 0;
269
270	cache->cur = 0;
271	if (swap_slot_cache_active)
272		cache->nr = get_swap_pages(SWAP_SLOTS_CACHE_SIZE,
273					   cache->slots, 1);
274
275	return cache->nr;
276}
277
278int free_swap_slot(swp_entry_t entry)
279{
280	struct swap_slots_cache *cache;
281
282	cache = raw_cpu_ptr(&swp_slots);
283	if (likely(use_swap_slot_cache && cache->slots_ret)) {
284		spin_lock_irq(&cache->free_lock);
285		/* Swap slots cache may be deactivated before acquiring lock */
286		if (!use_swap_slot_cache || !cache->slots_ret) {
287			spin_unlock_irq(&cache->free_lock);
288			goto direct_free;
289		}
290		if (cache->n_ret >= SWAP_SLOTS_CACHE_SIZE) {
291			/*
292			 * Return slots to global pool.
293			 * The current swap_map value is SWAP_HAS_CACHE.
294			 * Set it to 0 to indicate it is available for
295			 * allocation in global pool
296			 */
297			swapcache_free_entries(cache->slots_ret, cache->n_ret);
298			cache->n_ret = 0;
299		}
300		cache->slots_ret[cache->n_ret++] = entry;
301		spin_unlock_irq(&cache->free_lock);
302	} else {
303direct_free:
304		swapcache_free_entries(&entry, 1);
305	}
306
307	return 0;
308}
309
310swp_entry_t get_swap_page(struct page *page)
311{
312	swp_entry_t entry, *pentry;
313	struct swap_slots_cache *cache;
314
315	entry.val = 0;
316
317	if (PageTransHuge(page)) {
318		if (IS_ENABLED(CONFIG_THP_SWAP))
319			get_swap_pages(1, &entry, HPAGE_PMD_NR);
320		goto out;
321	}
322
323	/*
324	 * Preemption is allowed here, because we may sleep
325	 * in refill_swap_slots_cache().  But it is safe, because
326	 * accesses to the per-CPU data structure are protected by the
327	 * mutex cache->alloc_lock.
328	 *
329	 * The alloc path here does not touch cache->slots_ret
330	 * so cache->free_lock is not taken.
331	 */
332	cache = raw_cpu_ptr(&swp_slots);
333
334	if (likely(check_cache_active() && cache->slots)) {
335		mutex_lock(&cache->alloc_lock);
336		if (cache->slots) {
337repeat:
338			if (cache->nr) {
339				pentry = &cache->slots[cache->cur++];
340				entry = *pentry;
341				pentry->val = 0;
342				cache->nr--;
343			} else {
344				if (refill_swap_slots_cache(cache))
345					goto repeat;
346			}
347		}
348		mutex_unlock(&cache->alloc_lock);
349		if (entry.val)
350			goto out;
351	}
352
353	get_swap_pages(1, &entry, 1);
354out:
355	if (mem_cgroup_try_charge_swap(page, entry)) {
356		put_swap_page(page, entry);
357		entry.val = 0;
358	}
359	return entry;
360}